In recent years, a system of transmitting and recording such digital data as music information and image information by connecting a plurality of AV equipment through a digital filter for example at home has been gaining ground. For example, video camera, Digital Versatile Disk (DVD) (trade mark) and other AV equipment with an interface according to the IEEE (The International of Electrical and Electronics Engineers, Inc.) 1394 high-performance serial bus standard (hereinafter referred simply as the IEEE 1394 serial bus) which is a digital bus are being developed.
Normally, movie data are copyright information, which requires some measures to prevent any unauthorized users from copying.
In order to prevent any copy by unauthorized users, for example, in the Minidisk (MD) (trade mark) system, a method called “SCMS (Serial Management System)” is used. This means information transmitted via a digital interface along with music information. This information indicates that the music data are either “copy free,” “copy once allowed” or “copy prohibited” data. On receiving music data from the digital interface, a minidisk recorder detects SCMS and if the data are copy prohibited, the music data are not recorded in a minidisk, and in case of copy once allowed data, the SCMS information is converted into copy-prohibited data and is recorded along with the music data received. In the case of copy-free data, the SCMS information is recorded as it is along with the music data received.
In this way, the minidisk system uses SCMS to prevent any unlawful copy of copyright data.
And in a data transmission system in which a digital interface is used to transmit or record music data, image data and other digital data, a method of storing reproduction control information in the packet header of each data packet on the transmission channel for transmission has been devised.
Such reproduction control information is defined by two bits for example as follows:
00: Copy free;
10: Copy once allowed;
01: No more copy allowed;
11: Originally copy prohibited.
On receiving a data packet, the recording equipment records the data and examines the reproduction control information. When the reproduction control information is either “01” or “11” representing the prohibition of copy, the data received are not recorded. And when the reproduction control information is “10” representing copy allowed only once, the reproduction control information is converted into “01” representing no more copy allowed and then the data received are recorded in the recording media.
In this way, the copy generation resulting from the original data is confined.
Further, in order to give the confining method of copy generation a coercive power, a method in which data are enciphered for transmission and the information required for enciphering and deciphering is licensed only to those suppliers who have entered an agreement to manufacture solely the equipment fully complying with the copy generation confinement system is also used.
Incidentally, the method of storing the reproduction control information in the packet header of data packets on the transmission channel for transmission leaves the possibility of the reproduction control information being changed by other apparatuses in the process of transmission of packets from a transmitting apparatus to a receiving apparatus.
For example, as shown in FIG. 6, even if the transmitting apparatus 1 transmits “11” meaning copy prohibited for the reproduction control information contained in the packet header of data packets, when the reproduction control information meets a changing attack during its transmission and the reproduction control information changes into “10” indicating copy once allowed, the data receiving apparatus 2 having received the packet cannot detect that this data is originally copy prohibited, and records the data as the reproduction control information contained in the packet header allows a one-time recording.
Thus, the traditional data transmission method is likely to become unable to control copy generation.
By the way, such two-bit information as mentioned above can represent only four types of reproduction control information. Depending on type of service, copy is allowed for up to two generations in addition to the four types mentioned above requiring to indicate a wider variety of reproduction control information, which in turn requires to use a larger number of bits.
However, the system of storing reproduction control information in the header of transmission packets for transmission leaves only a limited reserved area in the header making it difficult to store many bits there.
There is another method of storing reproduction control information not in the packet header but within the data. In this case, the difference of position and meaning of the reproduction control information by data format requires that the receiving apparatus to search for the reproduction control information for each format and to interpret the meaning thereof.